The present invention relates to switching systems for communications systems, and in particular to an improved switching system which includes a pair of three-stage connecting networks and a method of operating the same.
Many communications systems, particularly telephone systems, contain a connecting network. A connecting network is an arrangement of switches and transmission links through which terminals connected to sources and terminals connected to destinatinos may be connected together in many combinations.
Calls in progress in a connecting network do not arise in a selected order or time sequence. Requests for connections (new calls) and terminations of connections (hangups) occur more or less at random, and the performance of a network when subjected to random traffic is measured by the fraction of requested connections that cannot be completed, or by the probability of blocking.
The performance of a connecting network for a given level of traffic is determined largely by its structure, i.e., by what terminals have a switch placed between them and can be connected together by closing the switch. The structure of a network determines what combinations of terminals can be connected together simultaneously, and if the structure is too simple, only a relatively few calls can be in progress at the same time. If the structure is extensive and complex, it may accommodate a large number of simultaneous calls in progress, but the network may be expensive to build and difficult to control.
Connecting networks made of square switches have long been used in telephone central offices. In a network made of square switches, each switch has the same number of inlets as outlets. The network is said to be arranged in stages if the switches are partitioned into sets called stages, two of which respectively carry the inlets to and outlest from the network, while the other are internal stages between the two, with the arrangement being such that every path from an inlet to an outlet passes through each stage only once. If the network is full access, each switch in a given stage has a link to every switch in the adjacent stages.
A network is in a blocking state if there are an idle inlet to and outlet from the network, but no available path exists through the network to connect the inlet and outlet. A routing algorithm may be used for choosing routes through the network for unblocked calls during its operation, or in the alternative to reestablish paths through the network in a manner to prevent the network from going to a blocking state. Such a network is said to be nonblocking in the wide sense if it is capable of blocking, but there is an algorithm for assigning routes to incoming calls that precludes any blocking, such that if the rule is applied, no blocking state is accessible. The system can then satisfy demands for connections as they arise without rejecting any or rerouting existing calls. However, networks controlled by such algorithms and the algorithms themselves are often complex.
A connecting network may also be nonblocking in the strict sense, if no matter what state the network may be in, it is always possible to connect together an idle pair of terminals without disturbing calls already in progress. In most new telephone switching systems, nonblocking in the strict sense is achieved by an N.sup.2 type of network. An example of this would be a 2000 port network which requires 4 million switch elements to ensure nonblocking, but such networks are quite complex and extensive and add considerable cost to the system.